FAKRO roof window blinds are designed and manufactured to fit FAKRO windows. The information on your ID plate (shown below) contains all the information you need to order the correct blind size for your window. In the example below the size of 77/118 means this window requires a 78cm x 118cm blind from the roof window blind range here.
It is not necessary to measure any part of the window before ordering a blind, this measurement is all that's required.
If your window is a custom size (meaning a custom size blind is required) the measurement will also be on the ID plate. You can discuss this measurement with the RW4Y Digital Team who will be able to provide you with a cost and place the item online for ordering.